1. Field of the Invention
The present invention relates to a semiconductor device including a field-effect transistor and a method for manufacturing the semiconductor device.
2. Description of the Related Art
Transistors used for most flat panel displays typified by a liquid crystal display device or a light-emitting display device are formed using a silicon semiconductor such as amorphous silicon, single crystal silicon, or polycrystalline silicon provided over a glass substrate. Further, transistors formed using such silicon semiconductors are used in integrated circuits (ICs) and the like.
In recent years, attention has been drawn to a technique in which, instead of a silicon semiconductor, metal oxide exhibiting semiconductor characteristics is used for transistors. Note that in this specification, metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor.
For example, a technique is disclosed in which a transistor is manufactured using zinc oxide or In—Ga—Zn-based oxide as an oxide semiconductor and the transistor is used as a switching element or the like of a pixel of a display device (see Patent Documents 1 and 2).
Meanwhile, it has been pointed out that hydrogen behaves as a supply source of carriers particularly in an oxide semiconductor. Therefore, some measures need to be taken to prevent hydrogen from entering the oxide semiconductor at the time of forming the oxide semiconductor, and the amount of hydrogen in an oxide semiconductor film or a gate insulating film in contact with the oxide semiconductor film is reduced to suppress change of threshold voltage (see Patent Document 3).